Auditory System Flashcards
How is the ear divided and where is it embedded in the body?
Outer, Middle, Inner ears
Organ is embedded in the petrous portion of the temporal bone = the hardest bone in the body
What are the functions of the outer eat (3)?
- To capture sound and to focus it onto the tympanic membrane
- To amplify some frequences by resonance in the canal
- To protect the ear from external threats
What is the main function of the middle ear and how is this done? (2)
Amplification:
- focusing vibrations from the large surface area of the tympanic membrane to a smaller surface area f the oval window
This change in surface area means pressure increases
Using leverages from the incustapes joint to increase the force on the oval window
What is the hearing part of the inner ear?
Cochlea
What is the function of the cochlea?
Function is to transduction of vibration to nerve impulses
while doing so it also produced a frequency / pitch and intensity / loudness analysis of the sound
What three compartments does the cochlea contain?
Scala vestibuli and Scala tympani:
these are bone structures that contain perilymph which is high in sodium
and Scala Media: Membraine structure containing endolymph which is high in potassium
Where is the hearing organ = organ of Corti located?
Inside the Scala Media found in the cochlea
It lies in the basilar membrane
What is the Basilar membrane?
It is found in the cochlea
Arranged tonotopically ( similar to xylophone )
so it is sensitive to different frequences at different point along its length
What hairs does the organ of Corti contain?
Hair cells:
Inner hair Cells = IHC
Outer hair cells = OHC
What lies on top of the Hair cells?
The tectorial membrane
Allow the hair deflection
Why is hair deflection important in the ear?
Allows depolarisation of the hair cells
OHC are in direct contact with the tectorial membrane and act as a connection between the membrane and IHC
if sound is too soft and mechanical movement is not enough the OHC cell will contract bringing the membrane lower to the IHC ( as these r the ones needed for hearing )
if the sound is loud enough it can depolarise IHC itself
What is the function of IHC?
Carry 95% of the afferent info of the auditory nerve
Function is the transduction of sound
into nerve impulses
What is the function of OHC?
Carry 95% of efferents of auditory nerve
Function is modulation of the sensitivity of the response
What are stereocilia?
The hairs on the Hair cells
Describe the method of Transduction?
The deflection of stereocilia towards the longest cilium will open K+ channels
–> Depolarisation of the cell and neurotransmitter is released
–> Louder sound with larger amplitudes with cause greater deflection of stereocilia and K+ channel opening
What do the spiral ganglions at each cochlea do?
Project via auditory vestibular nerve (VIII) to the ipsilateral cochlear nuclei (monoaural neurones)
Where does the auditory info from each ear cross over?
Superior olive level near brainstem
after the superior olive level all connections are bilateral
How is hearing tonotopically organised?
Difference frequences of basilar membranes apex vs base are represented by different nerve fibres. e.g. apex nerve fibres take low frequences
this is taken up accordingly to primary and secondary auditory cortex
What safety mechanism is in place if sounds are too amplified and can cause discomfort?
OHC can elevate so the tentorum membrane does not touch the IHC as much and induce less depolarisation
Less loudness senses
: active amplifier
- if outer hair cells do not work the range of hearing will decrease e.g. someone screaming will sound like a whisper and very loud noise will be too loud
What is frequency?
Measured in Hz (20-20,000)
Cycles per second, perceived tone
What is Amplitude?
Measured in dB (0-120)
Sound pressure, subjected attribute correlated with physical strength
Why is the decibel scale useful?
It is a log scale, useful because range of sensitivity is very large
How does hearing change over time?
Hearing acuity decreases with age, particularly higher frequencies
How is a hearing assessment done?
- tunning fork
- audiometry
- central processing assessment
- tympanometry
- atoacustic emission
- electrocochleography
- evoked potentials
How is a Tunning fork used to test hearing?
Establish presence of absence of a hearing loss with a significant conductive component
Includes Weber test and Rinne test
What is a PTA?
Pure tone Audiometry:
science of measuring hearing acuity for variations in sound and frequency
Audiometer can produce sound
How is a audiogram plotted?
hearing thresholds are plotted to define if there is a hearing loss or not
Normal threshold is 0-20dB
what does Central processing assessment test?
Assessment of hearing abilities other than detection
verbal and non verbal tests
e.g. sound localization, filtered speech, speech in noise
What does tympanometry test?
Examination used to test condition of middle ear and mobility of the tympanic membrane and conduction bones
By creating variations of air pressure in ear canal
- check slide 25
What is Otoacoustic emissions?
Normal cochlea produces low intensity sounds called OAEs
these sounds are produced from the OHC themselves as they expand and contract
test part of newborn hearing screening and hearing loss monitering
What are auditory evoked potentials?
- Electrocochleography - evoked by clicks or tone burst. electrical activity from cochlea and 8th nerve
- Auditory brainstem response - evoked by clicks. activity from 8th nerve and brainstem nuceli + tracks
- Late responses - evoked by tone burst and oddball paradigm. activity from primary auditory and association cortex
Auditory brainstem response is used more often why?
Does not require attention from px and alteratiosn in the latency of waves can point to the location of the deficit,
ABR also used in babies and children
What are corticol potentials?
affected neurological conditions or processing problems
What is conductive hearing loss?
problem in outer or middle ear
What is sensorineural hearing loss?
problem in inner ear or auditory nerve
What is Mixed hearing loss?
conduction and transduction of sound affected
How can hearing loss be classified?
Mild , moderate, severe, profound
30-40
40-70
70-90
90-120 (dB)
What are causes of hearing loss in Outer and middle ear?
Outer ear: wax + foreign body
Middle ear : otitis* + osteosclerosis
Cerumen impaction of tympanic membrane
*Otitis is when bubbles can be seen through ear drum = liquid in the middle ear
What are causes of hearing loss in Inner ear and nerve related?
Inner ear: Presbycusis + Ototoxicity
Nerve : Nerve 8 tumour
What do hearing aids do?
Amplify the sound, do not replace any structure
What is a cochlear implant?
Replaces function of hair cells by receiving sound, analysing it and transforming it into signals
Sends electric impulse to nerve
Needs a functioning nerve to work
What is a brainstem implant?
If nerves are affected, the electrical signals can be sent to electrodes placed in brainstem
- v risky, advised for ppl with bilateral important auditory nerve damage
